Simplifying
(2a + 3b)(6a + 9b)

Multiply (2a + 3b) * (6a + 9b)
(2a * (6a + 9b) + 3b * (6a + 9b))
((6a * 2a + 9b * 2a) + 3b * (6a + 9b))

Reorder the terms:
((18ab + 12a2) + 3b * (6a + 9b))
((18ab + 12a2) + 3b * (6a + 9b))
(18ab + 12a2 + (6a * 3b + 9b * 3b))
(18ab + 12a2 + (18ab + 27b2))

Reorder the terms:
(18ab + 18ab + 12a2 + 27b2)

Combine like terms: 18ab + 18ab = 36ab
(36ab + 12a2 + 27b2)